At present, the fardh duty of greatest importance is the Islamic Unity.
The Damascus Sermon (90)

O brothers who are listening to these words of mine here in the Umayyad Masjîd! And O Muslim brothers in the masjîd of the world of Islam forty to fifty years later! Do not make excuses, by saying, "We do no harm, but we do not have the power to give benefit; therefore, we are excused." Such an excuse will not be accepted. Your laziness and displaying no effort by saying, "What is it to me?" and your failure to display a spirit of energy through Islamic unity and true Islamic nationhood are the grievous harm you cause, and it is injustice.
The Damascus Sermon (55)

The prescription for a magnificent, unfortunate continent, a glorious, defeated state, and a valuable ownerless nation is Islamic Unity.
The Damascus Sermon (109)

Important risales and compilations on the subject:
THE ISLAMIC UNITY - 1 - Compilation
The Twenty-Second Letter - The Risale on Brotherhood (Ukhuwwah)
The Twentieth Flash - The Risale On Ikhlas
The Twenty-Eighth Letter-The Sixth Matter, which is the Sixth Risale - On the domination of Haramayn Sharifayn by the Wahhabis
The Fourth Flash - The Highway of the Sunnah – On Shi’a
The Muslim-Christian Alliance - Compilation